Magnetic antenna systems



h 7, 1956 A. A. HEMPHILL 3 3 MAGNETIC ANTENNA SYSTEMS Filed Jan. 3, 1952PIC-3.2

MAGNETIC MATERIAL.

MAGNETlC,

MATERIAL l4 ll \ls MAGNETIC MATEQlAL.

I8 I] A? INVENTOR.

AL FRED A, HEMPHILL.

2,740,113 MAGNETHI ANTENNA SYSTEMS a lication January 3, 1952, SerialNo. 254,717 3 Claims. 01. 343-787) Thisinvention relates generally toantenna systems and more particularly to improved arrangements ofmagnetic antennas which are suitable for use as antennas on mobile craftand other applications Where protruding dimensions must be minimized oreliminated.

'The art of signaling by means of electromagnetic radiation, in thepast, has resulted in the advent of a wide variety of antennaarrangements for transmitting or receiving thewave energy signals. Theseantennas, While varying, greatly in detail, are almost universally ofthe type which. can be described as a conductivity discontinuitydisposed in the transmission medium of free space and operating usuallywith reference to a conducting ground plane, For antennas in the form ofa conducting electric. element the distance between the element and theground plane is required to be quite large for satisfactory operationand the effective height (i. e. the ratio of terminal voltage to fieldstrength) rapidly approaches zero as this distance decreases. Forantennas in the form of a slot in the ground plane efficient operationcan be obtained without any physical extension from the ground planeThese latter antennas, however, are generally only practical for therelatively short wavelengths, such as in the microwave region.

The present invention is directed to the provision of flush mountedantennas which have no projection from the ground plane with which theyoperate and yet maintain' a useable eifective height as signaltranslators. This ample, the aircraft beacon band in the neighborhood of200 fkilocycles and below, without undue physical size by employing amagnetic antenna element which is embedded in an opening in theconducting ground plane. The antenna so formed has been found to produceresults which are at least as good as prior art electric antennas and byvirtue of the physical arrangement thereof aiford considerably improvedstructural features, such as the aerodynamic advantages in aircraftapplications or the like.

A particular system in which the magnetic antenna of the presentinvention undesirable interference with the air flow over craft surfaceand for increasingly higher speeds Z,740,l l3 Patented Mar. 27, 1956 ofprotrusion that can be tolerated becomes vanishingly small. Thisrequirement of smooth surfaces for uninterrupted air flow is in directopposition to the requirement of a sensitive radio frequency electricantenna, inasmuch as the sensitivity of such antennas is, in general, afunction of the projection thereof normal to the aircraft surface. Thephysical arrangements of such antennas are further restricted inasmuchas the protruding portion of the antenna in direction finderapplications is, in general, required to be rotated.

It is, accordingly, a primary object of this invention to provide a newand improved antenna system which is operable when fiush mounted in aconducting surface.

Another object is to provide a directional antenna suitable fordirection finding applications which may be operably located on anaircraft without protruding from the surface thereof.

Still another object is to provide a flush mounted magnetic antennasystem which can be flush mounted on a conducting surface and which issimple and economical to construct and exhibits high sensitivity andreliability in operation.

These and other objects of the invention are accomplished according tothe present preferred embodiment thereof by mounting a magnetic antennaelement, such as a dust iron or ferrite bar, in a conducting surface orclosely adjacent thereto and suitably coupling signals from the coilwound thereon. As utilized in a direction finder system the inventionprovides a plurality of radially disposed bars of powdered iron or othersuitable high frequency permeable material. The radial bars aredepressed in a conducting surface having incident thereon anelectromagnetic radiation field. The coplanar bars and mounting surfacetherefor provide an ideal aerodynamic surface and such an arrangement,in accordance with the teaching of the present invention, is withoutdeleterious effect antenna. The bars are disposed with respect to acentral opening in which a disc core member made of a similar permeablematerial is mounted for rotation. The

disc core has wound thereon a coil: which has a voltage induced thereinin accordance with the flux induced in the radial bars and the relativeposition of the bars and the coil. The relative position at which a nullis secured is indicative of the direction of arrival of theelectromagnetic field.

For the purpose of this specification and the appended claims,high-frequency high-permeability materials and magnetic antenna elementsare to be understood to mean materials which have high-resistivity andpermeability, and magneticantennas constructed with such materials. Suchmaterials are those known, for example, as ferrites or such compositionsas dust iron. formed into suitable solid shape by a high-resistivitybinder. It will further be understood. that conducting-surface or thelike is hereby limited. to mean non-magnetic conducting materialsurfaces which have low-permeability, for e x' an antenna. system inaccord magnetic flux in the bar by means of, for exon the electricalperformance of the a mounted in a. fragmentary Fig. 3 is a sectionalview along the line 3-3 of Fig. 2; and

Fig. 4 is a sectional view along the line 4-4 of Fig. 2.

Referring now to Fig. 1, there is shown a conducting surface 8 havingsecured in an opening therein a magnetic rod 9 which has a coil 10 woundthereon. The rod 9 has a high-permeability and is non-conducting and mayhave a dielectric constant greater than unity, if de sired. One ferritematerial which has given satisfactory results is that known in the tradeas Stackpole Ceramag 4. The antenna may be utilized for transmission orreception by connecting the terminals of the coil 10 to appropriatecircuits in a conventional manner.

In Fig. 2 is shown an aircraft surface 11, which has embedded thereinthree bars 12 which are made of a high-permeability high-resistivitymaterial or iron dust held together with a suitable non-conductingbinder. The bars 12 are long in comparison to their cross-sectionaldimensions and are retained in the recesses in the surface 11 by beingembedded in low loss plastic 13 or like material which may be molded andhardened. Rotatably mounted in a circular opening 14 in the surface 11and in a hub position with respect to the rods 12, is a disc 15 made ofhigh-frequency permeable material similar to that of the bars 12. Thedisc 15 has a coil 16 wound thereon which may be center-tapped, ifdesired. Connections to the coil 16 are made by means of slip rings aswill be hereinafter described. The rods 12 may have shaped pole shoeswhich are adjusted to reduce a sextantal error in the null voltageposition of the coil 16 resulting from the three rod arrangement.

In Fig. 3 the bar 12 is shown embedded in plastic 13 thus forming asubstantially continuous surface between the portions of the aircraftsurface 11 adjacent the recess. if desired, an exactly flush surfaceconstruction could be used.

Fig. 4 shows the core 15 rotatably mounted in a hearing 17 coaxial withthe opening 14. The end and center terminals of the coil 16 areconnected to slip rings 18 which are contacted by brushes 19. The coiland core assembly may be rotated by manual or power means, not shown,depending upon the particular system into which the antenna isincorporated. In order to obtain a suitable inductance value it may bedesirable to provide the core 15 of a thickness somewhat greater thanthat of the rods 12. This may be achieved without undue loss in the fluxtransferred from the rods 12 to the core 15 by providing suitably shapedpole shoes 21. The opening 14 may be closed by a suitable cover platemounted in a recess, if desired.

As is evident in Fig. 2 the air gaps between the pole shoes 20 or 21 andthe core 15 are considerably smaller than those between adjacent polepieces. With three bars of magnetic material disposed and proportionedas shown,

where eachbar opposes a virtual or phantom bar which is the resultant ofthe other two, and where the pole faces each embrace only about at mostof the circumference of the core, while the coil is widely distributedover the core, the flux paths about the coil have always essentially thesame reluctance, thus minimizing inductance variations. The large airgap between adjacent pole faces of the bars, as compared to the gapbetween the pole faces and the core, reduces shunting to a negligiblefigure. For a given effective antenna height this structure is extremelyli ht.

The operation of the magnetic antenna system of the present inventionmay be qualitatively explained by analogy with the well known electricantennas. As hereinbefore described, conductive antennas, such as thehalfwave dipole, represent a conductivity discontinuity and theperformance of such antennas is markedly affected by the proximity ofother conductors such as the ground plane. The magnetic antennarepresents a magnetic discontinuity of high permeability and inaccordance with the present teaching the performance thereof is notseri- 22 which may be flushously impaired by the proximity of theconducting ground plane. In the case of the electric antenna theanalogous question of the eifect of the presence of non-conductingpermeable bodies has apparently been of insuflicient practical interestto warrant investigation; whereas, the present teaching of the utilityof a magnetic antenna located in a conducting surface is eminentlypractical as, for example, in high speed aircraft installations.

In the operation of the direction finder of Fig. 2 the antenna system ofthe present invention picks up radio frequency signals with an effectiveheight equivalent to that of many prior art protruding antennas. Theeffective height of the antenna increases with physical dimensions fordimensions much smaller than a wavelength and in a particularapplication it will be understood that the maximum physical size andweight of an antenna will be fixed by other considerations. For example,a satisfactory di rection finder antenna similar to that of Fig. 2 forthe frequency range of -1750 kilocycles per second had the bars 12 eachof length 14 inches, and 0.75 inch in diameter, thereby providing aphysical arrangement well adapted for mounting in the wing of anaircraft. The'bars 12-may be designed as magnetic antenna elements in amanner sim ilar to that described in an article entitled, The MagneticAntenna by Leigh Page, published in Physical Review, June 1946. For thispurpose, the bars 12 should be as long as is practicable and thetransverse dimensions suit: ably chosen. By making the air gap betweenthe ends of the rods 12 and the core 15 small, substantially all of theflux from the rods 12 will pass through the 'core 15 and induce avoltage in the coil 16. The voltage induced in the coil 16 will have apattern with respect to rotation of a figure 8 similar to well knownloop patterns, which may be utilized in any conventional manner forindicating the direction of arrival of the electromagnetic waves. Byvirtue of the absence of any projection into the air-stream, the antennasystem of the present invention produces no aerodynamic disturbances.

Many modifications of the antenna system here described will be apparentto those skilled in the art in the light of the above teaching. Variouscore and coupling arrangements are possible by analogy to various othermagnetic circuits. In the direction finder antenna, changes may be made,such as a greater number'of radial rod members may be used to improvethe symmetry of the antenna aperture relative todirection. In someapplications, it may be desirable to adjust the angular positions of oneor more of the radial bars relative to the others to compensate forquadrantal error of the aircraft.

What is claimed is:

l. A direction finding magnetic. antenna system for electromagneticradiation comprising, three rods formed of particles of magneticmaterial embedded in a high-resistivity binder and having lengths whichare several 'tirnes longer than their cross-sectional dimensions, meansdisposing said rods radially in the field of said radiation, a core ofhigh-frequency highpermeability material rotatably mounted centrally ofsaid rods, a coil on said core, and means for obtaining a signal fromthe induced voltages in said coil, the air gaps said rods beingsubstantially greater than the air gaps'b'e tween said rods and saidcore, whereby shunting of flux and inductance'variations are minimized.

2. A direction finding magnetic antenna system tromagnetic radiationcomprising,

for elecradially in a conducting plane with an essentially: equi angularrelationship and exposed to said radiation,

3. A magnetic antenna system comprising, a plura'lity of between theadjacent ends of three rods of highfl frequency high-permeabilitymaterial disposed essentially.

flux and inductance variations high-frequency high-permeability antennaelements, means References Cited in the file of this patent for radiallydisposing said elements essentially in a con- UNITED STATES PATENTSducting surface, a rotatable core centrally located with respect to saidelements, a coil covering substantially the 1,842,347 Eaton 19321,844,859 Levy Feb. 9, 1932 full width of said core, and signalfrequency means 5 coupled to said coil, the air gaps between theadjacent ends 2242300 Woods May of said elements being substantiallygreater than the air 2266454 Wagstafi? 94 gaps between said elements andsaid core, whereby shuntfi g 3 5 2 mg of flux and inductance variationsare mmlmized. 10 2:581:348 Bailey Jan. 1952 FOREIGN PATENTS 874,946France Aug. 31, 1942 726,143 Germany Oct. 7, 1942

